There is a need to provide compact, lightweight, inexpensive coherent light sources in the entire visible spectrum i.e., visible lasers. A further convenience would be to use commercially available efficient diode lasers for pumping these visible lasers. The available He-Ne lasers (632.8nm) and visible diode lasers (650 mm) made from quaternary semiconducting materials are in the red region of the spectrum only and further, are either too weak or too narrow bandwidth of emission or too large to meet most of the application requirements. Applications of compact visible lasers include medical diagnostics, surgery, metrology, optical storage, display, HDTV, submarine-satellite communication and other areas. Efficient high power diode lasers (more than 20 Watts of quasi-cw power) are commercially available in the infrared and near infrared wavelengths (zero dispersion with very low propagation loss in optical fibers) for large scale applications in optical communication. The attempts so far, in upconverting these diode lasers have been in the areas of efficient cavity designs for second harmonic generation or very low temperature (.about.50 degrees Kelvin) laser operation of rare earth ion doped solid state materials, often involving expensive nonlinear crystals and elaborate mechanical and optical alignments. The research on dye doping in solid host materials for waveguide lasers (J. C. Altman et al. in IEEE Photonics Technology Letters, Vol. 3, No. 3, p189, Mar., 1991 and references therein) have had very limited success due to photodegeneration of dyes which are not recycled as in a liquid dye laser.
The energy level diagram and absorption/emission of a typical dye molecule (Rhodamine 6G) is shown in FIGS. 1A and 1B. The singlet (total electron spin=0) and triplet ( total electron spin =1) manifold of electronic energy states with the associated rotational-vibrational levels of a dye molecule is shown with a linearly increasing energy scale as the ordinate. The ground energy state of the dye molecule is designated by the symbol S.sub.O the singlet state of lowest energy. It includes a range of energies determined by the quantized vibrational and rotational states of the molecule. The energy spacing between the vibrational states range from 1400 to 1700 cm.sup.-1. The rotational state spacing is about two orders of magnitude smaller than the vibrational states. A near continuum is thus formed.
When light is absorbed, the molecule is excited from the ground state to a vibrational-rotational level of an excited state S.sub.1 or higher. After excitation, a thermal distribution of the population takes place within 10.sup.-11 seconds among the continuum of sublevels. Molecules can also be excited to the second excited singlet state S.sub.2 by either (1) absorption of a photon by an already excited molecule in the S1 state or (2) absorption of a single photon (usually ultraviolet) directly by a molecule in the ground state or (3) two-photon absorption of a molecule in the ground state. Nonradiative decay then occurs from the S.sub.2 state to the S.sub.1 state in a time of 10.sup.- 10 seconds.
In other words, as dye molecules are currently understood, once they are in an excited state (either S1 or S2), they relax thermally (i.e., nonradiatively) to the low lying vibrational-rotational levels of the S1 state in a maximum time of typically 10.sup.- 10 seconds (Karl H. Drexhage, Chapter 5 of "Dye Lasers", Ed. F. P. Schaefer, Topics in Applied Physics, Vol.1, Springer-Verlag). From what has been reported so far, a few molecules (less than 5%) undergo intersystem crossing and energy is transfered to excited triplet states. Triplet states have been known to be metastable with microsecond or longer lifetimes. Thus, molecules caught in triplet states for such long times are a major source of loss of efficiency of dyes as a laser medium. This is basically photobleaching or photodegradation of dyes.
A molecule in the S.sub.1 state can then return to S.sub.O state by emitting a photon whose energy is less than the absorbed light. This S.sub.1 .fwdarw.S.sub.O radiative transition gives rise to spontaneous and stimulated emission in laser dyes. The spectrum of a typical absorption and emission of a dye molecule is shown in FIG. 1B. The high rate of spontaneous emission from S.sub.1 .fwdarw.S.sub.O with radiative lifetimes of the order of nanoseconds is responsible for the high gain in dye lasers, often several orders of magnitude larger than solid state lasers.
The principle used in the waveguide lasers discussed in the above-mentioned article of Altman et al has been conventional, i.e., being pumped at the absorption maximum of the dye which then lases in the longer wavelengths. Structure and properties of laser dyes have have been very well documented by Karl H. Drexhage in the above mentioned reference.
Low loss optical waveguides offer the potential for propagating high intensity beams (intensity &gt;100 MW/cm.sup.2) over long distances. Waveguide materials with strong nonlinear absorption and minimum linear absorption offer good potential for efficient upconversion. If the energy levels responsible for upconversion fluorescence allow population inversion then by use of any suitable feedback such as holographic gratings or mirrors at the end of the waveguide, one can achieve an integrated upconverted laser. This is the principle of multiphoton pumped upconverted laser, named as MU-LASER.